There is a need for staring imaging systems capable of real-time high-speed imagery in a large number of spectral bands. (A staring image is a two-dimensional image that is captured at a single point in time, such as a camera image. A scanning image is a one-dimensional image that is scanned over different points in space as a function in time to form a 2-dimensional image). A staring system would be useful for determining if momentary spectral changes occur in a two-dimensional field of view. Such momentary changes could easily be missed by a scanning system.
Existing spectral imaging technologies do not support high-speed imagery because mechanical scanning mechanisms are too slow. Techniques that employ electronic scanning methods and two-dimensional focal planes are required. In addition, techniques that support changing spectral sampling on-the-fly are able to match changing target and background conditions, particularly when it is necessary to optimize signal-to-noise ratio (SNR). In addition, under certain noise conditions, spectral multiplexing techniques have the potential of increasing SNR and decreasing the sample rate for a given measurement.
Multiplexing is well-known in spectroscopy because of its ability to improve SNR. One of the available multiplexing techniques, Hadamard Transform Spectroscopy (HTS). An excellent review of HTS and its mathematical derivations is available in M. Harwit et al., Hadamard Transform Optics, Academic Press, 1979.
The optical multiplexing advantage of Hadamard transform spectroscopy is due to a weighing design scheme. By simultaneously measuring multiple wavelength intensities according to a weighing scheme or Hadamard masking order, a corresponding increase in accuracy is observed. Two types of weighing schemes are available. The Hadamard H-matrix weighing design consists of entries corresponding to −1, 0 and +1. The simplex or S-Matrix mask utilizes +1 and 0 in the weighing design. Due to the relative ease in transforming the Hadamard mask encoded data, the additional requirement for a left-cyclic rotation of the S-matrix for each order is common for optical applications of a Hardamard weighing scheme.
FIG. 1A represents a left-cyclic Hadamard S-Matrix weighing design with an order of 7. A black square indicates an off or ‘0’ condition, while a white square is an on or ‘1’. In the example, each position in the Hadamard mask corresponds to specific wavelength intensity. The resulting 7 observables are multiplied by the inverse of the Hadamard S-matrix, as shown in FIG. 1B, to solve for the individual wavelength intensities. Assuming that detector noise is independent of the amount of light reaching the detector, the SNR improvement approaches   ∼                    n            2        .  If every nth observable consisted of measuring the intensity at a single wavelength (i.e., the weighting scheme had 6 off positions and 1 on position), the system would be equivalent to a conventional dispersive spectrometer. However, using the example of FIG. 1, the multiplexing advantage is illustrated because for the same seven observables each wavelength intensity is measured four times instead of once. For the Hadamard order of 7, the SNR improvement is negligible. Typical Hadamard orders implement in an actual instrument are 1 to 2 orders of magnitude higher than the example shown in FIG. 1.
Hadamard transform spectral imaging approaches have the potential to achieve the aforementioned improvements in SNR. Significant work in this area has been reported by a team at Kansas State University where a digital micro-mirror device (DMD™) from Texas Instruments (TI, see L. Hornbeck, U.S. Pat. No. 5,535,047) has been utilized to implement a Hadamard transform spectrometer (HTS). (See R. A. DeVerse et al., Spectrometry and imaging using a digital micromirror array, American Laboratory, October 1998, pp. 112S–120S; R. A. DeVerse et al., Hadamard transform Raman imagery with a digital micro-mirror array, Vibrational Spectroscopy 19 (1999) 177–186; R. A. DeVerse et al., An improved Hadamard encoding mask for multiplexed Raman imaging using single channel detection, Journal of Molecular Structure 521 (2000) 77–88; and W. G. Fateley et al. Modulations used to transmit information in spectrometry and imaging, Journal of Molecular Structure (550–551 (2000).) The paper describe the use of a DMD™ (or DMA) as a spatial light modulator for generating a stationary Hadamard encoding mask. The DMD™ in a dispersive flat-field spectrometer was utilized as a 1D Hadamard mask for spectral encoding. Due to the relatively high cost of 2D multichannel detectors in the near and mid infrared, the Raman imaging systems of DeVerse et al employ a single element detector for imaging in the visible spectral region. A 1D Hadamard cyclic S-matrix encoding mask (spectral encoding) is folded into a 2D Hadamard encoding mask and, together with sample rastering a single element detector, can be used for Hadamard transform imaging. While alluding to economic reasons, these papers do not discuss the technical problems that impede the development of viable staring 2D imaging system using a 2D detector and encoding the spectral dimension.
Prior art systems have been developed and proposed that utilize Hadamard encoding of the spatial dimension using a DMD™ or 2d spatial light modulator (SLM) and a 2D detector. The systems encode one spatial dimension by focusing the input image on the DMD™ or SLM and passing the encoded images through a spectrograph (Q. S. Hanley, et al, Spectral Imaging in a Programmable Array Microscope by Hadamard Transform Fluorescence Spectroscopy, Applied Spectroscopy, Vol 53, No.1, 1999) or utilize a Fourier Transformed Infrared light source (T. J. Tague, Jr., et al., U.S. Pat. No. 5,923,036).
The DMD™ has a 1024×768 matrix of discrete, rectangular mirrors that are supported on two opposing corners and which may be controllably tilted between a first position where one free corner is down and the opposite free corner is up and a second position where the one free corner is up and the opposite corner is down. The range of motion of each mirror is −10°.